The present invention relates to a method of, and apparatus for, reducing non-linearity (if any) present in the frequency-versus-time characteristic of the output signal of a swept-frequency radar transmitter.
Swept-frequency radar apparatuses which transmit an output signal which has a substantially linear frequency-versus-time characteristic during each frequency sweep are well-known. If the current output signal is mixed with a version of said output signal which has been transmitted and reflected back by a target a beat signal results, because the frequency of the output signal will have changed during the time taken by the transmitted output signal to travel to the target and back again. The frequency of the beat signal arising from reflection by a particular target will therefore be representative of the range of that target if the velocity of the target relative to the radar apparatus is zero, or will be representative of a combination of the range and the relative velocity of the target otherwise, because any non-zero relative velocity will give rise to a Doppler frequency shift in the reflected signal. Thus analysis of the beat signal into its respective frequency components will yield range/relative velocity information about any target which has given rise to a reflection.
It will be appreciated that a given target at constant range will only give rise to a constant beat frequency if the frequency-versus-time characteristic of the radar transmitter output signal is accurately linear. Any increase or decrease of the (positive or negative) rate of change of frequency during a given frequency sweep will give rise to an increase or decrease respectively in the frequency of each frequency component of the beat signal during that sweep, effectively resulting in the spreading of each frequency component and hence in a decrease in the resolution of the range/velocity information contained therein.
In order to optimize the frequency sweep linearity it is known from, for example, a paper "On performance of a linear FM radar transmitter at 35 GHz" by P. Z. Peebles and A. H. Green at pages 4-10 of Proc. IEEE Southeast Con. 1982 to employ a closed-loop feedback system to provide dynamic correction. To this end the output signal of the radar transmitter is applied directly to one input of a mixer and also to the other input of the mixer via a delay line having a small delay relative to the repetition period of the (periodic) frequency sweeps of the output signal. The resulting output of the mixer is a beat signal which ideally (if the frequency-versus-time characteristic of the output signal is perfectly linear) has a constant frequency f.sub.o equal to the product of the delay produced by the delay device and the rate of change of frequency of the output signal. This beat signal is applied to what is in effect a frequency discriminator tuned to f.sub.o and the discriminator output signal is combined with the (sawtooth) modulation waveform which produces the frequency sweeps in the output signal in such a sense as to reduce any deviation of the frequency of the beat signal from f.sub.o. Thus any non-linearit y in the frequency-versus-time characteristic of the output signal, which non-linearity gives rise to a deviation in the frequency of the beat signal from f.sub.o, results in a modification to the modulation waveform such as to reduce the non-linearity. The control loop in effect continuously maintains the rate of change of frequency of the transmitter output signal at a value such that the frequency of the resulting beat signal is f.sub.o.
A disadvantage of the known feedback system, at least in some potential applications, such as automotive radar, is the cost and/or bulk of the additional microwave or high-frequency components required thereby, e.g. the delay line and the means by which the delay line is coupled to the remainder of the apparatus.